1. Field of the Invention
The invention relates to apparatus for a dynamic thermal-mechanical material testing system that not only self-resistively heats and deforms a specimen, under controlled conditions, but also substantially reduces adverse effects in specimen performance, such as magnetically induced motion or non-uniform specimen heating, that would otherwise result from magnetic fields which impinge upon the specimen and are caused by high heating currents flowing in the apparatus. This reduction is achieved by spatially locating high current carrying conductors used in the apparatus such that these conductors collectively generate substantially balanced, i.e. substantially equal, and opposite magnetic fields that effectively cancel each other out in a volumetric region of the apparatus which contains the specimen and particularly its work zone.
2. Description of the Prior Art
Metallic materials play an indispensable role as an essential component of an enormous number of different products. One crucial property of such materials is their ability to conduct electricity. Absent operation at superconductive temperatures, a metallic object possesses a resistance to electrical current flow based upon its cross-sectional size, length and resistivity. Owing to this resistance, the object will generate heat whenever an electric current is passed therethrough. This form of heating is the so-called "self-resistive heating". Self-resistive heating finds use in a wide number of diverse applications.
Different materials, including those that are metallic, possess widely varying mechanical, metallurgical and other properties. As such, the specific properties required of a material for use in a given application are first determined followed by selection of a specific material that exhibits appropriate minimum values of these properties. An essential step in selecting a specific material is first to determine its properties of interest by testing specimens of each such material being considered.
Materials are tested in a wide variety of different ways. One such way, which is experiencing substantially increasing use, is dynamic thermal-mechanical testing. Here, a specimen is held between two opposing anvils or gripped at each of its two ends in a jaw system. The specimen is typically in the form of a small cylinder or sheet section of a given material and has a substantially uniform circular, rectangular or square cross-sectional area. An electric current is serially passed from one anvil (or jaw assembly) through the specimen and to the other anvil (or jaw assembly) to generate, through self-resistive heating, a rapid, but controlled, heating rate throughout the specimen. Self-resistive heating is used inasmuch as it can produce very high material temperatures, such as in excess of 3000 degrees C., that are only limited by the melting range of the material. While the specimen is being self-resistively heated, various measurements are made of the specimen. Depending upon the specific measurements being made, the specimen either may or may not undergo controlled deformation while it is being heated. If the specimen is to be deformed, then this deformation can be accomplished by moving one of the two anvils (or jaw assemblies), at a controlled rate with respect to the other, in order to impart, e.g., a controlled compressive or tensile force to the specimen. Physical measurements, such as illustratively specimen dilation and temperature, are typically made while heating and deformation are simultaneously occurring. This testing not only reveals various properties of the specimen material itself, such as its continuous heating transformation curve, but also various dynamic properties, such as illustratively hot stress vs. strain rates and hot ductility; the dynamic properties being particularly useful in quantifying the behaviour of the material that will likely occur during rolling, forging, extrusion or other material forming and/or joining operations. One system that provides excellent dynamic thermal-mechanical testing is the GLEEBLE 2000 system manufactured by Duffers Scientific, Inc. of Poestenkill, New York (which also owns the registered trademark "GLEEBLE" and is the present assignee). This system advantageously heats the specimen in a manner, using direct self-resistive heating, that is expected to generate transverse isothermal planes throughout the entire specimen. Specifically, since each specimen generally has a substantially uniform transverse cross-section throughout its length, then, for low frequency current, the current density is expected to be uniform throughout the entire specimen which will cause uniform heating over the entire cross-section.
In order to produce the requisite level of self-resistive heating throughout the specimen, relatively high currents, typically on the order of several thousand amperes or more, must be passed through the specimen to produce a desired heating rate and specimen temperature. The amount of this current generally depends upon a number of factors, for example: the specific heat of the material; its resistivity; the geometric shape of the specimen, such as its cross-sectional area and length; heat loss from the specimen to its surroundings, principally including but not limited to the anvils (or jaw assemblies); and the value of the final temperature to be attained. In practice and owing to the low resistances of most specimens, generally only a few volts or less need to be applied across the specimen to conduct the required heating current therethrough.
Within such a testing system, the heating current must be routed through suitable conductors between a power supply, frequently a transformer secondary, and both anvils (or jaw assemblies). These conductors frequently take the form of either flexible wire of an appropriate gauge or, as in the GLEEBLE 2000 system, so-called "rolling flexible conductors" that contain a number of copper strips that have been laminated together. In either case, these large currents generate appreciable magnetic fields around the conductors.
The applicant has discovered that since a portion of these conductors is often situated in the vicinity of the specimen, then, during heating, a significant non-uniform magnetic field is produced by these conductors which extends throughout a volumetric region occupied by the specimen. This field tends to adversely affect specimen testing, and specifically specimen performance, in two ways. First, this field induces non-uniform current flow in the specimen that, when combined with the uniform current density established by the heating current flowing through the specimen, causes the total current density to vary throughout the specimen. This, in turn, causes undesired local variations in the temperature of the specimen. For specimens with relatively small cross-sectional area, these variations remain small and are generally insignificant. However, as specimens of increasing cross-sectional area are used, such as a 10 mm bar, these variations correspondingly increase and can be quite noticeable. These variations also increase and can become quite pronounced as the specimen undergoes heating and grows in width as the result of a simultaneously occurring compressive deformation. As such, it is well known in the art of material testing that as large specimens are used, these specimens must be properly positioned within a dynamic thermal-mechanical material testing system such that a relatively constant distance, particularly during the course of compressive deformation, can be maintained between the specimen and all high current conductors. Unfortunately, since such a position greatly depends upon the particular specimen being used, e.g. its geometry and magnetic properties, and the specific amount of deformation it will encounter, determining such a position has proven to be quite tedious and often difficult in practice. Second, the non-uniform field, typically occurring at a 50 or 60 Hz power line frequency, induces mechanical motion in the specimen, specifically causing it to noticeably vibrate. This motion, if it occurs with a sufficiently large amplitude, can generate substantial stress in the specimen, which, in turn, can induce unwanted strain therein, i.e. a change in material shape. The strain, if sufficiently large, can corrupt certain test results. In addition, this stress can be particularly troublesome if the specimen is to be heated in a stress-free manner. In this regard, ferrous specimens, with a relatively large surface area and if heated with sufficiently large currents, can disadvantageously exhibit significant amounts of induced motion. Since, in general, such materials, typified by many steel alloys, are extremely important from a commercial standpoint, it is imperative to obtain test results from a thermal-mechanical material testing system that are as accurate as possible for these materials.
Thus, a need exists in the art for a dynamic thermal-mechanical material testing system, and specifically for apparatus for inclusion therein, that substantially eliminates the adverse affects on specimen performance which would otherwise result from appreciable non-uniform magnetic fields that impinge upon the specimen and are generated by the high heating currents flowing through the apparatus.